Ultrasonic wave propagation, signal attenuation and signal interface loss in the human body are a function of frequency. For imaging ultrasound, the sound wave frequency must be higher than that of a sufficiently short wavelength to enable imaging resolution of the scattered and reflected signals. This requirement limits testing in the human body to areas that can be penetrated by higher frequency ultrasonic signals usually above 1 MHz. Among other things, there is a need to extend the range of ultrasonic applications to medical diagnostics that will not be limited by imaging wavelengths resolution requirements.
Ultrasonic measurements can be performed in a range of methods and applications. In medicine, major effort is placed in ultrasonic imaging where ultrasonic echoes are analyzed forming 1D, 2D or even 3D images of the target signal reflections. The ultrasonic test frequencies for imaging are in general above 1 MHz with an upper range of 20 MHz or more. Volume test coverage is achieved with ultrasonic transducer movement or the wobble of the transducer axis ultrasonic beam. Most modern imaging instruments use array transducers and employ phase array ultrasonic signal processing. With very few notable exceptions, transducers or phase arrays are driven by controlled shape electrical impulses. Ultrasonic signal reflections, time gating and image formation in medical high-frequency ultrasound explores extensive signal processing algorithms including distance amplitude correction, frequency filtering, Doppler signal extraction, signal threshold corrections, noise reduction correlation methods and more.
In therapeutic ultrasonic applications, as opposed to diagnostic applications, ultrasonic signal frequencies are often below 1 MHz and the transducers emit range of sound intensities and signal waveforms including r.f. type signals and customized impulses (r.f. or r.f. signal as used herein means a multi-cycle ultrasonic signal with certain duration and frequency content). None of these applications are usable for the diagnostic ultrasonic measurements.
Except for a possible few notable exceptions, ultrasonic diagnostic equipment has a mono-static (pulse echo) configuration where transmitter and receiver transducers are collinear in the test space. Bi-static diagnostic measurement configurations have not been considered and there is no practical ultrasonic equipment that uses physically separated transmitter and receiver transducers. Bi-static configurations are extensively used in the ultrasonic nondestructive testing technologies where through transmission (opposing transmitter and receiver transducer set) is commonly used in ultrasonic test configurations.
In conventional ultrasonic imaging applications, all transducers are coupled and in contact with the test object. Using contact transducers in mono-static geometry, above 1 MHz has enabled the development of a broad array of useful ultrasonic test configurations capable of imaging internal organs, fetus and other medically interesting targets.
Acoustical and ultrasonic waves at lower frequencies, e.g. 25 kHz to 1 MHz are used in other applications including seismology, underwater acoustics and nondestructive materials testing. At these lower frequencies more recent developments of non-contact air coupled transducers make possible ultrasonic testing without the need to contact the test object.
As always, there is a need for new, different, and/or improved methods of testing and diagnostics in the medical profession. Accordingly, employing ultrasound at relatively low frequency domains (e.g. 25 kHz to 1 MHz) and low power levels in a testing or diagnostic fashion fills a need in the art, as does the development of apparatus, systems, and methodologies to facilitate such use. Applicants have invented such apparatus, systems and methods employing low frequency ultrasound as a non-imaging test and/or diagnostic.